1. Field of the Invention
The present invention relates in general to digital signal processing and in particular, to digital tone controls and systems using the same.
2. Description of the Related Art
Most audio equipment includes tone controls allowing adjustment of the audible treble and bass responses as a matter of listener taste. In low-cost applications, such as inexpensive portable consumer audio appliances, where precise tone adjustment is not a critical requirement, minimal tone controls can be implemented with simple analog circuits and a small number of buttons, dials or knobs. In higher-end consumer appliances and professional audio equipment, the size, complexity and cost of the tone controls increase in proportion to such factors as the number of frequency bands over which precise control is desired and the number of steps of attenuation needed in each frequency band.
With the maturing of several digital audio technologies such as Compact Disks (CDs), Digital Video Disks (DVDs) and MPEG Layer 3 audio, the prevailing trend has been to perform more of the audio processing functions in the digital domain. Among other things, it would be clearly be desirable to implement the tone control functions in the digital domain. This would eliminate the need for specific analog circuits, such as resistor-capacitor OP-AMP filters, allow the tone controls to be implemented in either hardware or software, and support direct control by the system digital processors or controllers. Notwithstanding, the problem of implementing digital tone controls is not trivial.
One possible means of implementing digital tone controls would be to set up a cascade bank of digital filters, either in hardware or software, each for setting the gain (attenuation) of selected frequency band of the incoming signal. To change the gain of a given frequency component, the filter coefficients of the corresponding filter would then be appropriately adjusted to boost or cut the level of that component relative to the other frequency components. However, changing filter coefficients on the fly can result in the filter traversing regions of instability, depending on the configuration of the feedback loops and mismatch between the changing coefficients and the initial conditions. As a result, “clicks”, “pops”, “zipper noise” due to rapid change and similar discontinuities and artifacts can occur in the audible output of filter coefficients corresponding to user adjustment of controls.
In sum, new hardware, software and methods are required for implementing digital tone controls. These implementations should be applicable to digital signal processor-based systems, as well as discrete digital audio processing circuits and systems.